The invention relates to static power systems in general, and more particularly to uninterruptible power systems (U.P.S.) such as used on aircraft, for computer systems, or wherever a source of proper nature, voltage and/or frequency is not directly available and when continuity of service is essential.
An uninterruptible power system (U.P.S.) generally uses the assist of a battery which will, or automatically can, be switched in parallel with a direct current power supply obtained by conversion from the network through a rectifier arrangement, in case the latter would fail. The direct current power is then converted by inverter to alternating current supplied to the user's load. Typical of a system for the generation of alternating current by inverter is the apparatus disclosed in U.S. Pat. No. 3,491,282 of T. M. Heinrich et al in which is described a static inverter system generating a sinusoidal wave of improved quality and which is voltage controlled.
Fully-redundant UPS systems are used for the most stringent demands of service continuity to a critical load, or for large installations on the order of 500 KVA and larger. It is anticipated that even the most reliable inverter (or other part of the UPS system) will experience failure eventually, and that the system performance must be protected from that kind of failure. By operating inverters in parallel with one more unit than is necessary for load requirements, it is possible to remove a malfunctioning unit without upsetting the critical load. Also, any one unit can be removed from operation for servicing, inspection, or testing, whithout reducing the system capacity. With larger rated systems, above 250 KVA for example, inverters must be paralleled to achieve the desired capacity. As a result a greater number of components are involved, and the inverter reliability may be statistically reduced. The use of redundancy will more than compensate for this drawback, and at lesser expense with larger systems.
An object of the present invention is to provide balanced load sharing between parallel channels each including static controlled rectifiers for supplying direct current to inverters feeding a common load.
Another object of the present invention is to provide an uninterruptible power system having inherently balanced load sharing between parallel and redundant channels.